Art of cracking hydrocarbon oils



Aug 24, X93 7.,

E. W. lsoM ET AL ART OF CRAGKING HYDROCARBON OILS Filed June 5, 192B Mig@ Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE ART OF CRACKING HYDROCARBON OILS Application .lune 5, 1928, Serial No. 283,021

4 Claims.

p D method of operation which makes it possible to l" subject heavy hydrocarbon oils to relatively severe cracking conditions while minimizing the usually incident disadvantages of such operation, particularly those disadvantages due to overcracking, such as excessive production of pitch or coke or so-called carbon or excessive production of so-called fixed gases. For commercial purposes, certain stocks must be subjected to relatively severe cracking conditions if useful yields of gasoline, for example, are to 20 be obtained. For example, stocks consisting of components most of which have previously been subjected to cracking operations, so-called cycle stocks, tend to be somewhat refractory. Likewise, lighter stocks tend to be more refrac- 25 tory than heavier stocks. Further, by subjecting certain stocks to relatively severe cracking conditions, it is possible to produce gasoline character products of special value as components of motor fuel. This invention is of special value 30 and application in the production of gasoline or gasoline-containing products from such stocks or other stocks where the raw stock is to be subljected to relatively severe cracking conditions.

i In carrying out the present invention, the 3J raw oil is forced successively through primary and secondary stages in a heating zone under superatmospheric pressure and thence into an expansion chamber maintained under a pressure lower than that prevailing in the heating zone and from which vapors are taken olf and residue is discharged, the oil is heated to a high cracking temperature in the primary stage in the heating zone, a relatively cool solvent oil is 45 introduced into the oil passing from the primary to the secondary stage in the heating Zone lowering the oil temperature and the oil mixture is discharged from the secondary stage in the heating Zone at a cracking temperature lower than 55) that at which the oil is discharged from the primary stage in the heating zone. The raw oil stock is thus subjected to severe cracking conditions and to maintained cracking conditions w while over-cracking is avoided by limiting the time at which the oil is at the extreme high temperature and by minimizing the deposition of pitch or the like as the cracking continues.

The secondary stage in the heating Zone may be sub-divided and relatively cool solvent oil, or a series of relatively cool solvent oils, introduced 5 at a number of points through the secondary stage as described in the above mentioned prior application.

The raw oil is with advantage preheated, for y example, to an incipient cracking temperature, l0 before introduction into the primary stage in the heating zone. This preheating may be accomplished, for example, by heat exchange with heating gases escaping from the major heating operations.

The vapors taken olf from the expansion chamber may be condensed in toto to form a composite product which may then be fractionated in any convenient manner for the separation of a gasoline product, a solvent oil fraction or fractions and any heavier fraction or fractions, for example as described in the above mentioned prior application. These vapors, however, with advantage are subjected to direct fractionation for the separation of such several fractions.

The invention will be further described in connection with the accompanying drawing which illustrates one form of apparatus adapted for carrying out the invention. It Awill be understood that the accompanying drawing is largely diagrammatic and conventional; the apparatus is shown generally in elevation partly in section and with parts broken away.

The heating furnace illustrated comprisesl a series of heating flues in which are arranged banks of heating tubes connected to form three single continuous coils, a iirebox and a stack flue, all connected so that the heating gases from the rebox l pass successively through the heating ues 2, 3, and 4 'to the stack flue 5. Ducts, including suitab-le forcing means are provided to permit recirculation of a part of the heating gases escaping from flue 3 to flue 4 through flues 2 and 3 as shown at 6 and to permit recircula- 45 tion of part of the heating gases escaping through the stack flue 5 through flues 3 and 4 as shown at 1.

The heating furnace illustrated is the sole invention of Edward W. Isom, one of the joint applicants herein. This particular heating furnace is here illustrated because it is of special value and application in carrying out the processV of this invention, as will appear.

In carrying out the invention in the apparatus illustrated, raw oil is forced by means of pump 8 through preheating coil 9, cracking coil I0 and cracking coil II in succession and then into expansion chamber I2 through pressure regulating and reducing Valve I3. Residual oil or tar is discharged from expansion chamber I2 through line I4. Vapors are discharged from expansion chamber I2 through line I5 into the lower end of fractionating tower I6.

The fractionating tower I6 may be of any conventional type, for example it may be a so-called bubble tower. The fractionating tower` I6 and the expansion chamber I2 are with advantage thoroughly lagged or thermally insulated.

In the fractionating tower those components of the vapor mixture heavier than suitable as components of the desired product are condensed. This condensation may be effected by circulating all or a part of the raw oil through cooling coil I1 in the upper end of tower I6 on its way to preheating coil 9.or by the reintroduction into the upper end of tower I6 of part of the distillate product through line I8 by means of pump I9 or by both of these means. Vapors remaining uncondensed escape from the upper end of tower I6 through line 20 to condenser 2| arranged to discharge into receiver 22. The condensed product is discharged from receiver 22 through connections 23 and 24 and uncondensed vapors and gases through connection 25.

A heavy fraction, similar for example to the raw oil, may be recirculated through the cracking coils I l) and II in admixture with the raw oil. The heaviest fraction separated in tower I6, for example, may be introduced into the oil passing from Ythe preheating coil 9 to the cracking coil I0 from the lower end of tower IG through lines 26 and 21 by means of hot oil pump 28. By returning this oil fraction hot, it is unnecessary to preheat it. All or part of this fraction may be discharged through connection 29, the oil fraction in this event usually being passed through the cooler 3U. l

The raw oil or oil mixture is heated to a high cracking temperature in the cracking coil I0, the primary stage. As this hot oil passes from the cracking coil Ill to the cracking coil II, a relatively cool solvent oil is introduced through line 3I by means of pump- 32. A separate solvent oil may be supplied through connection 33, or with advantage an intermediate fraction or fractions separated in the fractionating tower I6 may be withdrawn through line 34 for this purpose. Such intermediate fraction or fractions may be passed through cooler 35 if necessary to maintain the desired conditions in the cracking coil I I, the secondary stage. Any deficiency of solvent oil within the system may be made up through line 33 and similarly any excess of solvent oil within the system may be discharged through line 33, and cooler 36 if necessary.

The solvent oils useful in carrying out the invention include generally those oils more refractory with respect to the cracking conditions maintained-in the operation than the raw oil to be cracked. For example, kerosene character stocks are useful as solvent oils in the cracking of gas oil character stocks.

, The following example of conditions of operation useful in carrying out the invention for the` production of gasoline from gas oil will illustrate the-invention. The discharge pressure from the cracking coil II may be in the neighborhood of SOO-500 pounds per square inch. Pressures as high as 1,000 pounds per square inch, however,may

be used in some cases. Pressure in the expansion chamber I2 may approximate atmospheric pressure, or the expansion chamber may be operated under superatmospheric pressures, for example 20-25 pounds per square inch, lower, however, than the discharge pressure from the cracking coil II. The fractionating tower I6 may be operated under substantially the same pressure as the expansion chamber I2 or, when the expansion chamber I2 is operated under superatmospheric pressure, the fractionating tower I6 may be operated under a pressure lower than that prevailing in the expansion chamber I2. Raw

oil may enter the preheating coil 9 through connection 31 at a temperature approximating 60- 80 F., although this temperature may be as high as 250 F. or higher if this raw oil has been Used as a refiuxing medium in the fractionating tower I6. Oil passing from the preheating coil 9 to the cracking coil I Il may be at a temperature approximating G-'100 F. 'Ihe heaviest fraction from the fractionating tower I6, if returned, may be supplied through connection 21 at a temperature approximating 50G-650 F. The oil may be heated in the cracking coil I0 to a. temperature, as discharged, approximating 850- 950 F. The solvent oil introduced into the hot oil passing from the cracking coil I0 to the cracking coil II may be supplied through connection 3l at a temperature in the general range of -400" F., and the resulting oil mixture may enter the cracking coil II at a temperature in the neighborhood of G50-800 F. The oil mixture may be heated in the cracking coil II to a temperature, asdischarged, approximating 80G-900 F. When the fractionating tower I6 is operated under substantially atmospheric pressure, the

temperature ofthe escaping vapors may be in the neighborhood of 350-400 F. It will be understood that the foregoing temperatures and pressures are given to assist in illustration of the invention and that the invention is not limited thereto.

The fractionating operation carried out in tower I6 is controlled by regulating the rate at which raw oil is circulated through cooling coil I1 or by regulating the rate at which the distillate product is reintroduced through line I 8. The furnace operation, that is the heating operation, is controlled by regulating the firing and by regulating the rate at which heating gases are recirculated through the ducts 6 and 1. When heating gases arewithdrawn from one point in a heating flue and reintroduced at a preceding pointin the same heating iiue there is a tendency, increasing as the rate of recirculation increases, for the temperature at the point of withdrawal to approach the temperature at the point of reintroduction at the same time that the temperature at the point of reintroduction is lowered. For this reason, in the heating furnace illustrated, the heating gases recirculated over the cracking coil I0 are withdrawn after passage over the cracking coil `II instead of before and heating gases which have also passed over the preheating coil 9 are separately recirculated over the cracking coil II. This makes possible the maintenance of closely uniform cracking temperatures throughout the cracking coil II and the heating of the oil to relatively high cracking temperatures in the cracking coil I0 while retaining the advantages of such recirculation of heating gases with respect to both cracking coils.

In the furnace illustrated, the oil is circulated through cracking coil I IJ generally countercurrent to the heating gases owing through ue 2, thereby promoting the heating of the oil to high cracking temperatures, and the oil is circulated through cracking coil ll generally concurrent to the heating gases flowing through flue 3, promoting the maintenance of the oil at a cracking temperature without excessive rise in temperature through this cracking coil,

The solvent oil supplied to the oil or oil mixture passing from the primary stage to the sec-- ondary stage in the heating zone, in carrying out this invention, acts in a dual capacity. It checks the decomposition of pitch or the like which might otherwise be deposited in contact with the heating surfaces, and any local overheating and consequent over-cracking involved in such decomposition of pitch or the like. over-active cracking induced by the relatively high cracking temperature to which the oil is heated in the primary stage in the heating zone as a cooling or diluting agent.

What is claimed is:-

l. In cracking heavy hydrocarbon oils to produce gasoline character products, the improvement which comprises forcing a restricted flowing stream of heavy oil successively through primary and secondary stages in a heating Zone under superatm-ospheric pressure and thence into an expansion chamber, heating the oil to a high cracking temperature upwards of about 850 F. as discharged in the primary stage, introducing a relatively cool solvent oil relatively refractory with respect to the oil supplied to the primary stage into the oil passing from the primary stage to the secondary stage in the heating Zone, heating the oil mixture in the secondary stage to a cracking temperature as discharged therefrom lower than the temperature of the oil as discharged from the primary stage and not exceeding about 900 F., reducing the pressure on the oil passing from the secondary stage in the heating zone to the expansion chamber, taking off vapors including vapors of the gasoline character product from the expansion chamber and dis- Y charging residue from the expansion chamber,

2. In cracking heavy hydrocarbon oils to produce gasoline character products, the improvement which comprises forcing a restricted flowing stream of heavy oil including raw oil and a heavy condensate fraction mentioned below successively through primary and secondary stages in a heating zone under superatmospheric pressure and thence into an expansion chamber, heating the oil to a high cracking temperature upwards of about 850 F. as discharged in the pri- It also checks pansion chamber, taking off vapors from the expansion chamber and subjecting them to a fractionating operation, taking off vapors including vapors ofr the gasoline character product from the fractionating operation, supplying a heavy condensate fraction from the fractionating operation to the primary stage in the heating zone, supplying an intermediate condensate fraction from the fractionating operation to the secondary stage in the heating zone, and discharging residue from the expansion chamber.

3. In cracking heavy hydrocarbon oils to produce gasoline character products, the improvement which comprises forcing a restricted flowing stream of heavy oil successively through primary and secondary stages in a heating zone under superatmospheric pressure and thence into an expansion chamber, heating the oil to a high cracking temperature upwards of about 850 F. as discharged in the primary stage, introducing a relatively cool solvent oil comprising an intermediate condensate fraction mentioned below into the oil passing from the primary stage to the secondary stage in the heating zone, heating the oil mixture in the secondary stage to a cracking temperature as discharged therefrom lower than the temperature of the oil as discharged from the primary stage, reducing the pressure on the oil passing from the secondary stage in the heating zone to the expansion chamber, taking off vapors from the expansion chamber and subjecting them to a fractionating operation, taking on vapors including vapors of the gasoline character product from the fractionating operation, supplying an intermediate condensate fraction from the fractionating operation to the secondary stage in the heating zone, and discharging residue from the expansion chamber.

4. In cracking heavy hydrocarbon oils to produce gasoline character products, the improvement which comprises forcing a restricted flowing stream of gas oil successively through primary and secondary stages in a heating zone under superatmospheric pressure and thence into an expansion chamber, heating the oil to a high cracking temperature upwards of about 850 F. as discharged in the primary stage, introducing relatively cool kerosene into the oil passing from the primary stage to the secondary stage in the heating zone, heating the oil mixture in the secondary stage Yto a cracking temperature as discharged therefrom lower than the temperature of the oil as discharged from the primary stage and not exceeding about 900 F., reducing the pressure on the oil passing from the secondary stage in the heating zone to the expansion chamber, taking oiT vapors including vapors' of the Agasoline character product from the expansion chamber and discharging residue from the expansion chamber.

EDWARD W. ISOM.

JOHN W. DANFORTH,

ROGER C. TOWNSEND, Eccccutors for Charles L. Parmelee, Deceased. 

